National causes can be made of small things - one thinks of Jenkins' Ear - but few can have had such a charming and witty source as the collection of small objects known as the Lewis Chessmen that have since the mid-19th century delighted visitors to the British Museum. The chessmen inspired the stories of Noggin the Nog; Harry and Ron Weasley played a game with replicas in the first Potter film. People take a great shine to them: the queens with their hands to their cheek looking so wise (or so bored), the wardens or rooks furiously biting their shields (the "berserkers", the soldiers of Odin).

Now they have been registered as a political grievance. Scotland's first minister wants them back. On December 19, Alex Salmond made a speech outlining the Scottish government's proposals to preserve the Gaelic language, and containing the following two sentences: "I find it utterly unacceptable that the Lewis Chessmen are scattered around Britain in a bizarre parody of the Barnett formula. And you can be assured that I will continue campaigning for a united set of Lewis Chessmen in an independent Scotland." This isn't quite the same thing as Captain Robert Jenkins showing off his pickled ear to the House of Commons in 1738, prompting a war against Spain that lasted nine years. Nevertheless, the war of the Lewis Chessmen threatens to run and run.

The comparison with the Barnett formula, which sets the level of Scottish subvention from the UK Treasury, is unclear, but the pieces are not "scattered around". The British Museum holds 67 chessmen and the National Museum of Scotland 11. They were made from walrus ivory in the 12th century, most probably in Norway. Chess had reached Norse civilisation not long before, after its slow journey from India to southern Europe via Persia and Arabia. Europe had humanised the abstract form of the Oriental pieces, even feminised one of them by turning the vizier into a queen, and military Christianity had replaced princes or "leapers" with bishops with mitres and croziers. To quote the excellent monograph written by a British Museum curator, James Robinson, the chessmen are "unique survivals ... no other visual record survives that documents so perfectly the full range and variety of arms and armour used in 12th-century combat". The likeliest speculation is that these beautifully crafted luxuries were on their way to princes or traders in the Norse-held territories of Ireland or the Isle of Man when, for reasons unknown, they were buried in the sand dunes of western Lewis.

The story of their discovery is also not reliably known. Most accounts give a crofter, Malcolm Macleod of Uig, as the man who brought them to a Stornoway merchant, Roderick Pirie, who took them to Edinburgh, where they were first displayed by the Scottish Antiquaries Society in 1831. The society wanted to buy them and preserve them as a collection, but the deal fell through. By this time an Edinburgh dealer, TA Forrest, had acquired them for £30. Forrest began negotiations with the British Museum. There were 82 pieces, and the museum was led to believe this was the extent of the hoard. But Forrest had already sold 10 pieces to an Edinburgh antiquarian, who then bought another stray bishop (these pieces now form the National Museum of Scotland's collection).

Neil MacGregor, the British Museum's director, knows of the demand only from reports in the press. Nobody from the Scottish government has been in touch. This month he emailed Linda Fabiani, Scotland's culture minister, to ask if the first minister's statement was an expression of his government's cultural policy. "Because if it is," says MacGregor, "we need to understand the principles that lie behind it." So far he has had no reply. (One can see Fabiani's difficulty. Shetland has been pressing strongly for the St Ninian's Treasure, a hoard of Pictish and Anglo-Saxon silverware, to be returned from the National Museum in Edinburgh. Last year Fabiani refused to support the Shetland claim, saying, "We have a hands-off approach to our cultural bodies, national collections and national companies, which is how it should be.")

Restitution of objects to where others think they belong is an increasing problem for many of the world's greatest museums, but the British Museum acquired its pieces legally and fairly and they are seen by many thousands of people from all over the word every year, for free. What could the arguments behind the chessmen claim be? That they can't be seen in Scotland? But they can. The British Museum lent many pieces to Stornoway's museum in 1995 and 2000, and is now in conversations about further loans. That they somehow embody "the spirit of the place"? A problematic argument, given their origins in Norway. That art and treasure best belong where they were found? As MacGregor says, "That would be a very alarming principle for Scotland's own public collections. Let's not forget that Scotland was part of a great imperial power and therefore has great imperial collections." A stroll around Scotland's largest museums would demonstrate his point. Is the Wylam Dilly, one of the two oldest locomotives in the world, to be returned to Newcastle from Edinburgh? Is Boulton and Watt's giant beam engine, built for a Southwark brewery in 1786, to go to Birmingham? Should all the beautiful grave-robbings go home to Egypt? If the rule were applied to Glasgow's Burrell collection, there would be nothing left, the ship-owner William Burrell having hopped around auctions like a magpie, picking up a Chinese pot here and a Norman arch there, including many items (it has to be said) from England. As for paintings, it is best not to go there. The place is heaving with Impressionists. The Bretons, some of them striving to be a nation, would have an excellent claim on Gauguin's Jacob Wrestling with the Angel, now resting at the National Gallery of Scotland instead of reminding Brittany so superbly of its patrimony and traditions. (And please don't mention the Elgin marbles, purchased by a Scotsman enriched by the profits of his Fife coalfield.)

It would be easy to accuse Salmond of nothing more than opportunism, adding to his reputation for that streak. In fact, he has been sporadically campaigning for the return of the Lewis Chessmen for 10 years. My explanation is that his demand comes out of a previous era of nationalism that was quite blind to Scotland's history as England's imperial partner - needed to be blind to it, because in terms of wealth it was Scotland's golden age and inconvenient to anti-English grievance. I had thought that the grievance mode was passing. But not yet, not yet. The idea has got about that England stole the chessmen and hid them in vault: "Give us back our wee men!"